Can a big left side varicocel, after long time cause right sided decrease blood flow? if yes will blood flow in right testis be normal after operation

Unlikely: Suggest you consult urologist for management of symptomatic large left-sided varicocele. Presence of associated discomfort is good indication that it shoul probably be corrected surgically. Options are: Laparoscopic left spermatic vein clipping (my preference), trans-abdominal vein ligation, microsurgical vein ligation thru scrotum or vein embolisation by interventional radiologist.